Laurel,
We just returned from Italy (yesterday) including Sorrento, Rome and Vernazza, we did Venice, Florence and Como on a previous trip. I agree with the earlier post about flying into one city and out of another (open-jaw) so you are not wasting time going back to your arrival airport, it also gives you some fare flexibility. On 10-12 days, I would fly in to Venice and spend 2 nights there, if you stay in the city, you have it almost to yourself in the evening as all the tours leave about 4:00. Start early and beat the crowds. There are fast trains to Rome and then on to Naples. In my opinion, Naples is a bust except for the Archaeology Museum which is a must see if you're going to do Pompeii (get a good guide, it's worth it). We stayed at the UNA Hotel in Naples mostly because it was convenient and we could check our bags while we visited the museum. For a treat, take the ferry across the bay of Naples to Sorrento which is worth 2 days max unless you're using it as a base to explore the area. Don't over do, it will wear you out. Positono and Amalfi would be 2 full days. Finally, I would head back to Rome and spend at least 3 full days, we did the Borghese one day, the Forum/Coliseum (4 hours for the 2 and it may be hot) and left the afternoons open for whatever we wanted to do, after the Forum/Coliseum when it was almost 100, a nap was in order. We stayed in an apartment 1 block off the Piazza Navonna, it was a great neighborhood and it had a washing machine...big plus when travelling so you don't have to pack for 12 days. The Rome airport is easy to navigate, we came in from Vernazza the afternoon before we left and stayed at the Hilton FCO to make it easier to catch our early flight, you can walk to Terminal 3 in 5-10 minutes. Good A/C and a true king-sized bed was nice too. I hope this helps, enjoy your trip.
Keith